Re: [PATCH v3 00/13] dax: fix dma vs truncate and remove 'page-less' support

[Christoph Hellwig]

On Fri, Oct 20, 2017 at 09:47:50AM +0200, Christoph Hellwig wrote:
> I'd like to brainstorm how we can do something better.
> 
> How about:
> 
> If we hit a page with an elevated refcount in truncate / hole puch
> etc for a DAX file system we do not free the blocks in the file system,
> but add it to the extent busy list.  We mark the page as delayed
> free (e.g. page flag?) so that when it finally hits refcount zero we
> call back into the file system to remove it from the busy list.

Brainstorming some more:

Given that on a DAX file there shouldn't be any long-term page
references after we unmap it from the page table and don't allow
get_user_pages calls why not wait for the references for all
DAX pages to go away first?  E.g. if we find a DAX page in
truncate_inode_pages_range that has an elevated refcount we set
a new flag to prevent new references from showing up, and then
simply wait for it to go away.  Instead of a busy way we can
do this through a few hashed waitqueued in dev_pagemap.  And in
fact put_zone_device_page already gets called when putting the
last page so we can handle the wakeup from there.

In fact if we can't find a page flag for the stop new callers
things we could probably come up with a way to do that through
dev_pagemap somehow, but I'm not sure how efficient that would
be.